In the process of manufacturing semiconductors or liquid crystals, thin substrates, which are substances to be processed like liquid crystal substrates or wafers, are stored in a storage cassette, and they are transferred between processing devices in the respective steps. The storage cassette is a box-like container that stores a plurality of thin substrates stacked, whose inner wall is cut to have a plurality of shelf-like grooves, and in which the thin substrates are inserted into the grooves, respectively, so as not to bring the stacked substrates into contact with each other.
When the storage cassette arrives at each step, a thin-substrate transfer robot carries out the thin substrates from the storage cassette and carries them into the processing device, and, when the processing is completed, the thin-substrate transfer robot stores the thin substrates in the storage cassette again. Generally, before extracting the thin substrates from the storage cassette, an inspection is made as to whether the thin substrates are stored in the cassette at a correct posture or not. The reason is that, if the thin substrates are stored at an abnormal posture like two-substrate insertion or oblique insertion, the thin-substrate transfer robot cannot extract the substrates from the storage cassette, and, if the substrates assuming such an abnormal posture are forcibly extracted therefrom, the storage cassette or the substrates will be damaged.
For this inspection, a method has been performed such that a transmission type or reflection type optical sensor using a laser beam or an LED beam is disposed in the vicinity of the storage cassette, and the state of the thin substrates in the storage cassette is detected.
However, a long-unsolved problem resides in that these optical sensors have difficulty in detecting a state (two-substrate insertion state) where two substrates are inserted in the same groove in two-ply form although the sensors can detect the presence or absence of the thin substrates. Another unsolved problem resides in that, since the detecting distance is short, the sensors must be moved in the vicinity of the thin substrates, and particles arise and contaminate the substrates. In recent years, an inspection method through image processing by use of a camera has been proposed in order to solve these problems.
FIG. 6 is a perspective view showing the structure of a thin-substrate inspecting apparatus that uses a camera.
In the figure, 1 denotes an inspection camera that is connected to an image processing apparatus not shown. 2 denotes a storage cassette in the interior of which a plurality of thin substrates 3 are stacked and stored.
The inspection camera 1 picks up an image of the thin substrates 3 stored in the storage cassette 2, and detects the state of the wafers 3 by processing this image. Since the inspection camera 1 performs photography while enlarging the limited field of view 4 so that the two-ply state of the thin substrates can be detected, the entire storage cassette 2 cannot be photographed at a time. Therefore, the camera 1 is moved in the upward/downward direction along an arrow 100 in order to detect the state of all thin substrates 3.
However, the following problems have arisen in using the camera.    (1) Since the field of view of the camera is limited, a driving device for moving the position and direction of the camera is required in order to see all wafers in the storage cassette. The camera having such a driving device interferes with a thin-substrate transfer robot because the camera requires large dimensions.    (2) If the camera and the driving device are disposed to avoid interference, the camera cannot be disposed at the most suitable position for inspection, and therefore inspection accuracy will deteriorate.    (3) Since much time is consumed to move the camera, inspection time becomes long.